text:
What are Runway Declared Distances?
Perusing through the A/FD in the United States I see that some runways have declared distances while others do not.
What is the meaning of:

TODA
TORA
ASDA
LDA

From the AIM, 4-3-6 (c)(2):

All 14 CFR Part 139 airports report declared distances for each runway. Other airports may also report declared distances for a runway if necessary to meet runway design standards or to indicate the presence of a clearway or stopway. Where reported, declared distances for each runway end are published in the Airport/Facility Directory (A/FD). For runways without published declared distances, the declared distances may be assumed to be equal to the physical length of the runway unless there is a displaced landing threshold, in which case the Landing Distance Available (LDA) is shortened by the amount of the threshold displacement.

Now for definitions. These terms are defined by the FAA/ICAO in the PCG.
TODA - Takeoff Distance Available. The takeoff run available plus the length of any remaining runway or clearway beyond the far end of the takeoff run available.
TORA - Takeoff run available. The runway length declared available and suitable for the ground run of an airplane taking off.
ASDA - Accelerate-Stop Distance Available. The runway plus stopway length declared available and suitable for the acceleration and deceleration of an airplane aborting a takeoff.
LDA - Landing Distance Available. The runway length declared available and suitable for a landing airplane.
(also Declared Distances from wikipedia)
which lead to other definitions...
Stopway - An area beyond the takeoff runway no
less wide than the runway and centered upon the
extended
centerline of the runway, able to support the
airplane during an aborted takeoff, without causing
structural damage to the airplane, and designated by
the airport authorities for use in decelerating the
airplane during an aborted takeoff.
Clearway - An area beyond the takeoff runway
under the control of airport authorities within which
terrain or fixed obstacles may not extend above
specified limits. These areas may be required for
certain turbine-powered operations and the size and
upward
slope of the clearway will
differ depending on
when the aircraft was certificated.
then for those specified limits of the clearway, in 14 CFR Part 1 we find

Clearway means:
(1) For turbine engine powered airplanes certificated after August 29, 1959, an area beyond the runway, not less than 500 feet wide, centrally located about the extended centerline of the runway, and under the control of the airport authorities. The clearway is expressed in terms of a clearway plane, extending from the end of the runway with an upward slope not exceeding 1.25 percent, above which no object nor any terrain protrudes. However, threshold lights may protrude above the plane if their height above the end of the runway is 26 inches or less and if they are located to each side of the runway.
(2) For turbine engine powered airplanes certificated after September 30, 1958, but before August 30, 1959, an area beyond the takeoff runway extending no less than 300 feet on either side of the extended centerline of the runway, at an elevation no higher than the elevation of the end of the runway, clear of all fixed obstacles, and under the control of the airport authorities.

A small correction to “take off run available” part. It’s not only the ground run but also may include some air distance over clearway. But this distance may not be longer than one-half of the distance between lift off point and point where aircraft reaches 35 feet AGL.
It’s introduced by SR-422A back in 1958 to utilize clearways for turbine powered aircrafts. This concept basically allows aircrafts to operate at higher gross weights otherwise limited by available runway surfaces only.

This is more appropriate as a comment.  What country are you referring to?  The definition for TORA came from the U.S. AIM 4-3-6.  What you described is the TODA distance listed above.

I got it from AC 25-7C take off section pg67-69. You’re right, AIM does not mention the air portion of it. Maybe they want to come up with a simplified version I guess. Or maybe I’m getting it wrong.

Thanks for the reference.  The "takeoff run" listed in the AC deals with the requirement for the airplane to reach 35 feet above the ground and it can use 1/2 the clearway to satisfy that requirement.  I would agree with that statement.  Declared distances, however, allow the airport to provide a safety margin for aircraft using that runway.  TORA values do not account for clearways.  TODA values do.  For airplanes that want to use a clearway, they will have a method of separating accelerate-stop and accelerate-go takeoff numbers (or be provided separately)

Yes, I agree. My intend was to clarify that the take-off run distance maybe longer than the actual concrete, TORA itself. That’s because you get credit for air distance part over the clear way. I can delete it if it’s not adding any value to discussion.


text:
Why is mist abbreviated BR on METARs and TAFs?
The abbreviation for a condition of mist is BR on METAR and TAF reports. How did mist end up getting abbreviated as BR?

According to a CFI's lesson plan on the internet, it comes from the French word for mist, brume.

I believe Smoke (FU) is also from the French (fumée)

as well as Hail (GR)  from French grêle.

The word "brouillard" would also fit and is more common, as far as I know. It's used on traffic signs in France.

Looked up the difference: brouillard is used when visibility is less than 1 km, brume is for visibility between 1 and 5 km.

"brume" is more used for mist while "brouillard" would be more often translated as fog (which would have reduced visibility).

A useful mnemonic for BR is "baby rain".

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
What is the true meaning of the word “wilco” when used on the radio? What is its origin?
Pilots often learn to use the word "wilco" on the radio through contextual interpretation. During my experience as a pilot communicating on the radio I have often observed pilots using the word "wilco" improperly. What is the true meaning of the word "wilco" and what is its origin?

"Wilco" is short for the phrase "will comply," meaning that the speaker will follow the instructions to which they are replying. Merriam-Webster places the origin at 1938, some time after the invention of radio, likely in military usage.
Although the phrase "roger wilco" is sometimes used, it is considered redundant since the "roger" (meaning "received and understood") is implied.

I think it's worth adding that "roger" and "wilco" should not be used together. Here's a reference.

@RyanBurnette Except in the case of Space Quest, where the protagonist is named "Roger Wilco". :)

@Steve I actually know the games and played them as a kid. Thanks for that! :)

@Steve I only realized how funny the characters name was after starting to fly

Damn. I've always wanted to say "Roger wilco, over and out." It's probably for the best.

In the US, the "official" meaning is in the Pilot/Controller Glossary:

WILCO − I have received your message, understand  it, and will comply with it

(Note that this is slightly different from roger, which is just an acknowledgement and doesn't mean that you will comply with anything.)
The AIM 4-2-3 mentions using it to acknowledge instructions:

Acknowledge  with  your aircraft identification, either at the
  beginning or  at the end of your transmission, and one of the words
  “Wilco,” “Roger,” “Affirmative,” “Negative,” or  other appropriate
  remarks

The problem I see with using wilco is that you're confirming that you'll comply with an instruction, but if you don't also read back the instruction then the controller has no way to know what you're complying with. You might have misheard or misunderstood it, so you should repeat it. But if you repeat it, then wilco is unnecessary. Outside the US, I was originally taught never to use roger or wilco for that reason: it leaves the controller wondering what you really heard and what you're going to do next. But obviously as far as US aviation goes, both are completely acceptable.
And as for the etymology, that's already been covered in another answer:

"Wilco" is short for the phrase "will comply," meaning that the
  speaker will follow the instructions to which they are replying

In the US, both roger and wilco are extremely rare. If a pilot is given a simple instruction like “Taxi to parking” they will often reply with just their tail number. Likewise with information like “Watch for birds at the departure end of the runway”. Instead of replying “Roger” you’ll normally hear just the tail number or something like, “I have the birds in sight.”

By syllable count, “roger” would be a much better response to a bird warning.

As other answers have already mentioned, WILCO is a concatenation of Will Comply.  To provide a UK perspective, there are a number of ATC instructions that must be read back, however in other cases "Wilco" is preferred.  The CAA Radiotelephony Manual CAP 413 states (my emphasis):

"Instructions transmitted are to be complied with and, in most cases, should be read back to reduce the chance of any ambiguity or misunderstanding, e.g. ‘G-ABCD, taxi to the apron via taxiway Charlie’. Chapter 2 specifies those instructions that are to be read back in full. However, if the instruction is short, clear and unambiguous, acknowledgment of the instruction using standard phraseology such as ‘Roger’ (I have received all your last transmission) or Wilco’ (I understand your message and will comply with it) is preferred for the sake of brevity in the use of radiotelephony transmission time."

Based on the rather strong opinions of a CAA examiner, the following scenarios illustrate an example of acknowledging instructions to report position:

G-ABCD, report abeam Farmoor reservoir"

If a read-back is given in the response and it is clipped, the controller could hear "... Abeam Farmoor reservoir G-ABCD", which could cause confusion.  Acknowledging with "Wilco G-ABCD", "Roger G-ABCD" or even just "G-ABCD" reduce the risk of confusion if the response is clipped, as well as providing brevity.

G-ABCD, number 2 in circuit, report final

In this scenario, again a clipped read-back could cause confusion ("... Final G-ABCD"). However, "Roger G-ABCD" (or just "G-ABCD") simply mean the transmission has been received.  This could refer to the traffic information, the instruction, or both.  The instruction could perhaps have been missed, especially as part of a more lengthy and complicated transmission.
In both scenarios, "Wilco" is the only response that unambiguously conveys that the instruction element of the transmission has been received and understood, and that it will be acted upon.  It is for this reason that it is preferred over "Roger" when a brief acknowledgement is desirable.

I have been a pilot for 47 years.  The use of the word WILCO vs. reading back a clearance is defined. Keep in mind that early radios were not easy to understand and the phonetic alphabet was formed. An ATC instruction "Plan on crossing XXXXX at FL280" can be addressed as WILCO as it is not a clearance.  "Cross XXXXX at FL280" is a clearance and must be read back.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What is the difference between deice and anti-ice?
Most aircraft contain systems which are used to mitigate problems which could be caused by ice. This includes everything from a heated pitot tube to a wing leading edge heated by bleed air. These systems are usually referred to as either deice or anti-ice systems.
What is the difference between deice and anti-ice?

De-ice is a system that removes ice that has formed on a part.
Anti-ice is a system that prevents ice from forming on a part.
A heated leading edge(bleed air) is an example of an anti-ice system.
A inflatable boot on a leading edge is an example of a de-ice system.

Does this mean that if ice has already formed that anti-ice is generally ineffective?

Certain systems can do both I think. e.g. for the chemical fluids sprayed on the wings there's a different dose / composition I recall for the de-ice application versus an anti-ice application.

@curious_cat ground deice uses one of several types of fluid which are usually applied hot (to deice) and have a specified "holdover time" (for how long the fluid remaining on the wing provides anti-ice protection). That's a different situation than in-flight de-ice/anti-ice though.

@voretaq7 Thanks! That's a tricky situation sometimes. I recall reading incident reports where a de-ice application was requested but an anti-ice application was made (or maybe vice versa) and things can get dangerous later if it goes undetected.

As the name indicates, anti-icing systems prevent the formation of ice (aircraft surfaces) while the de-icing systems remove the ice after it is formed.
Anti-icing systems can be active or passive. In general, most of the active anti-icing systems are thermal, which use the engine bleed air routed through the pipes in wings, engine intakes etc.

Source: navyaviation.tpub.com
Another method of anti-icing is to have resistive circuits in wings, which can be generate heat when current is passed, preventing ice formation. This can be used also in de-icing mode to remove ice, which is more energy efficient compared to the anti-icing  mode.
One of the oldest de-icing method was the pneumatic one, which uses an inflatable rubber boot to remove accumulated ice from leading edges of wings.

"Deicer (PSF)" by Pearson Scott Foresman - Archives of Pearson Scott Foresman, donated to the Wikimedia Foundation→This file has been extracted from another file: PSF D-240001.png.. Licensed under Public Domain via Commons.


text:
How do I get permission to fly when diagnosed with Sleep Apnea?
I was diagnosed with Obstructive Sleep Apnea a few weeks ago.  I'm told that this somehow complicates my ability to get a license (I haven't started on my license yet.)  I'm having a hard time figuring out what the FAA page is trying to tell me (and what the accompanying "helpful chart" means.)
Could someone who speaks FAA style legalese give me a run down of what is being said?  I just want to know if I'm allowed to fly with treated sleep apnea, and what I need to do to make sure my medical is completed properly...

That FAA page and chart are for AMEs, not pilots, so they're not exactly user-friendly :-) If you aren't already a member, join AOPA: their members forums have lots of threads about this, and they can provide advice on the medical process too. Looking at their OSA page it seems that everything starts with an AME determining which risk category you're in.

When an Aviation Medical Examiner, AME, issues you a medical certificate, that is the relevant "permission" you need. Having sleep apnea complicates that issuance process, but doesn't necessarily make it impossible. Along with what you read on the AOPA forums, I would suggest making an appointment to talk with a local AME, preferably one who has accomplished this process with other pilots before. He can give you a picture of what the process is likely to look like in your case, which is a better answer than assuming that your journey will look like that of somebody you read about on the internet.

There is a huge percentage of the population with sleep apnea, and it's been a really big deal with the FAA.  I'm sure there is a standard procedure for getting cleared to fly with treated OSA.

An individual's specific symptoms could affect the specifics of how long it would take them to satisfy the FAA's requirements - thus the recommendation to talk to an AME for the answer in his case, rather than a generic answer that may or may not apply. Since the AME is the one who issues the medical, he is the authoritative source for what this pilot will need... not an internet message board, or "I'm sure there must be..." opinions.

Lemme expand on what I mean:  There is a document the FAA created trying to explain the hoops you need to jump through and what my diagnoses might mean, but I'm having trouble following it.  I'm looking for an interpretation of it that doesn't involve me scheduling an appointment and spending money to speak to an AME.  Does that make more sense?  If you'd like to discuss in more detail, please join me in chat: http://chat.stackexchange.com/rooms/12036/the-hangar

@JayCarr Paying for AOPA medical and consulting with them is going to be your best option for what you want. They will give you a point by point overview of the process and help get your medical through the process. Especially helpful if your medical gets deferred to OKC.

@casey Well, if you and Ralph J and Pondlife are all going to say the same thing...  Then I guess I'm just mistaken and maybe it's time to get an AOPA membership in lieu of getting a license.

Sorry for the pointless edit Ralph J, I did it so I could change my vote when I realized your answer is about as correct as I can expect.

I’ll add that if an applicant has any concerns about passing, they should book an “eval” appointment with an AME first. Do not book an actual “application” until you’re certain you’re going to pass.

Ralph J Covered it pretty well but I will expand on what he said with a few other facts.
In reality you only need one AME to sign you off. With that in mind if the first one says you are a no go that does not mean that its said an done. Another AME may have a different opinion and sign off on your medical. An AME may be able to give you a good feel if not a straight answer over the phone if you can accurately describe your condition. I see you have this tagged as PPL so I will assume you are going for you PPL and asking in that regard. If you are talking about an aviation career I am under the impression that some of the majors do not let you chose your own AME and may make you go to their in-house doctor. This may be an issue down the line if your local AME cleared you but the airline you eventually work for has a different opinion.
While still under debate, third class medical reform may change the playing field for all of this and you may no longer need a medical cert to fly private.
AOPA is a great resource for this kind of thing and they are one of the big parties advocating for third class reform.

If you have a disqualifying condition you still cannot fly with a drivers license after the law is changed.  See FAR 61.53.


text:
What are the guidelines for call signs for aircraft dealing with US ATC?
Suppose a general aviation aircraft is operating VFR under part 91. When they call ATC they'd normally use their tail number as the call sign. Something like "November 1-2-3 Foxtrot Uniform." What if they called up as "little piggy 27?" Are there guidelines or regulations about this? Does a custom call sign require anything or can you just make one up?

According to JO 7210.3Y, Air Traffic Organisation Policy, section 4-4-2 there are several requirements from both the FAA and FCC. The FAA requirements are (paraphrased from the JO):

Comply with FCC regulations (remember that the FCC is responsible for radio communication technical regulations, not the FAA)
Get FAA approval "to avoid possible duplication or conflict
with air−ground call signs assigned on a national
basis to other aircraft operators"

The FCC requirements are in 47 CFR 87.107 (not 87.115 as the JO says):

(a) Aircraft station. Identify by one of the following means:
(1) Aircraft radio station call sign.
(2) The type of aircraft followed by the characters of the
  registration marking (“N” number) of the aircraft, omitting the prefix
  letter “N.” When communication is initiated by a ground station, an
  aircraft station may use the type of aircraft followed by the last
  three characters of the registration marking. Notwithstanding any
  other provision of this section, an aircraft being moved by
  maintenance personnel from one location in an airport to another
  location in that airport may be identified by a station identification
  consisting of the name of the company owning or operating the
  aircraft, followed by the word “Maintenance” and additional
  alphanumeric characters of the licensee's choosing.
(3) The FAA assigned radiotelephony designator of the aircraft
  operating organization followed by the flight identification number.
(4) An aircraft identification approved by the FAA for use by aircraft
  stations participating in an organized flying activity of short
  duration.

In other words, your call sign must be approved in advance by the FAA, otherwise you can't legally use it per FCC regulations. In reality, if you call up ATC as Little Piggy 27 you'll either get a laugh and a "no, seriously?" or - in the worst case - you'll just annoy them.
There are also military call signs in use but I know nothing about them. I do remember being in the pattern while an Air National Guard aircraft out of Memphis, TN was doing touch and goes using the call sign Elvis 1. But I have no idea how that call sign was set up between the ANG and FAA.

I might as well add since you mentioned military callsigns, Civil Air Patrol (USAF Aux Not really a military organization) Aircraft callsigns are semi-permanent which go like: "CAP nnnn"

Operator call signs are generally established with the FAA>
In addition, operators can establish local call signs with their ATC facility.
E.g. News Chopper 10.
The FAA has established aircraft types in call signs at well.
You can't just make up a call sign.

Do you have any references?

Here's one on local call signs: http://www.faa.gov/documentlibrary/media/notice/final_n_jo_7210_878_use_of_aircraft_call_signs.pdf

http://www.faa.gov/air_traffic/publications/media/AIM_Basic_4-03-14.pdf P. 4-2-3

I think it would improve the quality of the answer to incorporate the references into the answer itself.

From ATC's point of view...
If a pilot calls up as "little piggy 27", the controller may respond initially with "little piggy 27".
When ATC establishes communication with a GA aircraft, they state the prefix "November" followed by the phonetic numbers/letters of the aircraft registration.
As a pilot, you could replace "November" with aircraft type, the model, or the manufacturer’s name, and ATC can respond in kind going forward.
[Info from JO 7110.65, 2−4−20. AIRCRAFT IDENTIFICATION]

authors: Александр Саркисян


text:
Is any endorsement required to take the Advanced Ground Instructor test?
What, if any, endorsement is required to take the Advanced Ground Instructor (AGI) test? 14 CFR 61.35 does make clear what is required for any knowledge test, but not what is specifically required for the AGI test.
I also noted that 14 CFR 61.193 mentioned that flight instructor privileges include providing an endorsement to take the test.
14 CFR 61.213 also lists several eligibility requirements to become and Advanced Ground Instructor including taking a knowledge test while mentioning no endorsement which is required to actually take the test.

No endorsement is needed. The AGI and IGI written tests can be done without anyone's endorsement.
Additions:

You also don't need an endorsement to take the CFI, CFII, or FOI written tests, btw.
Your reference to 61.193 has nothing to do with this. For one, it's 61.193, not 61.139 (but your link itself is correct), and second, these are not endorsements for written tests. In 61.193 it says "to train and issue endorsements that are required for: ..." and that does not include any endorsements for the AGI, IGI, FIA, FII, and FOI written tests. Such endorsements, as they pertain to a ground instructor, would, for example, be a 61.217(d) endorsement (not written test).
"... flight instructor privileges include providing an endorsement to take the test." No, they do not. They include providing an endorsement for a ground instructor certificate/licence (not written test), and endorsements for written tests (in general -- not for the ground instructor).

When an endorsement for the written test is needed, then that prerequisites say that explicitly, such as for the PPL, IR, CPL, ATPL, etc., such as in 61.65.(a)(4), for example.


text:
What is the true meaning of the word "roger" when used on the radio? What is its origin?
When pilots learn to talk on the radio they can generally learn that the word "roger," when used on the radio, is a way to acknowledge that a message was received. What is the textbook definition of the meaning of this word when used on the radio? Also, what is the origin of this word?

Obligatory link

@TomMcW Yesssss!

JAN phonetic alphabet / Able Baker alphabet successor of first alphabet by ITU (1920) where R was Roma.

In the US, the "textbook" meaning is in the Pilot/Controller Glossary:

ROGER − I have received all of your last  transmission. It should not be used to answer a  question requiring a yes or a no answer.

The AIM 4-2-3 gives an example of using it to acknowledge instructions:

Acknowledge  with  your aircraft identification, either at the
  beginning or  at the end of your transmission, and one of the words
  “Wilco,” “Roger,” “Affirmative,” “Negative,” or  other appropriate
  remarks; e.g., “PIPER TWO ONE  FOUR LIMA, ROGER.”

As a counterpoint to that, when I first learned to fly in South Africa I was told never to use Roger in transmissions because it doesn't tell the controller anything useful (obviously the FAA disagrees).
And as for the etymology, that's already been covered in another answer:

This usage comes from the initial R of received: R was called Roger in
  the radio alphabets or spelling alphabets in use by the military at
  the time of the invention of the radio

The word "roger" is used to acknowledge that a message was received, and goes back to the early uses of radio. From Wikipedia:

This usage comes from the initial R of received: R was called Roger in the radio alphabets or spelling alphabets in use by the military at the time of the invention of the radio

My understanding is "received and understood".  Is that a common interpretation?

I think so, it does not mean "I will comply" for which wilco is a shortening of. It just means you have heard and (mentally) made a note.


text:
Why does supersonic flight detach airflow from a wing?
I've read in several answers to questions that when a wing passes the speed of sound the airflow will become detached from the craft towards the rear quarter of the wing (thus making things like elevators ineffective.)
Why does this happen at Mach Speed?  Can it happen at lower speeds as well (in straight and level flight, ie, not stalled)?

Flow separation happens when the pressure gradient of the airflow along the flow path becomes too steep. In subsonic flow, the oncoming air is first decelerated ahead of the wing, then swiftly accelerated when it flows around the strongly curved nose section of a wing. This acceleration is the consequence of the wing's curvature.
After this acceleration, how will the airflow behave? Will the airflow keep moving in a straight line without further acceleration or deceleration? (which implies that airflow will separate from the wing surface, creating a local vacuum) Or will it faithfully stick to the wing surface? In reality, the air settles at a compromise between the straight path and following the contour, creating decreasing pressure along a surface with increasing curvature and increasing pressure along surfaces with decreasing curvature. More precisely, it is always in an equilibrium between inertial, viscous, and pressure forces.
This suction not only bends the airflow into following the wing's contour, but also accelerates the air ahead of it. The lower the pressure, the more the air speeds up, such that the total energy of air (the sum of pressure and kinetic energy) stays constant. Therefore, pressure and local speed change in sync.
When the curvature decreases further downstream, the flow path becomes straighter and pressure rises again. However, the air particles close to the wing slow down because of friction. The layer of air where this slowing is noticeable is called boundary layer. In it, the deceleration effects due to pressure rise and due to friction add up, and at some point the air comes to a standstill relative to the wing. Where this happens, static air will collect and build up, causing flow separation. Thankfully, the exchange of air across a turbulent boundary layer kicks the slowest air particles downstream, so at moderate angles of attack the air still moves along until it reaches the trailing edge. Only when the suction peak around the nose becomes very high at high angle of attack, the consequent steep pressure rise along the remaining flow path overpowers the possibilities of the turbulent boundary layer, the air decelerates completely and the flow separates. This is a wholly subsonic affair.
If the wing moves at high subsonic speed, the curvature-created suction accelerates the flow such that it reaches supersonic speed. Now something odd happens: Supersonic flow accelerates further when subsonic flow would decelerate. This is caused by the change in density which is dominant at supersonic speed. Incompressible (= very slow) flow has constant density, and all speed changes affect pressure. At Mach 1, the pressure and density changes are of equal magnitude, and in supersonic flow the density changes dominate. Now we have a supersonic pocket of air on the upper surface of the wing where speed increases and density decreases downstream, and the surrounding subsonic air sees little change in density. This picture should give you some idea how it looks:

The whole wing in the picture above moves at Mach 0.68. Compare the green color at some distance from the wing with the scale on the left side, which gives the Mach number for each color shade. At the airfoil nose, you see a blue area. This is where the air decelerates - it gets pushed together by the approaching wing. Now follow the colors along the upper side - they quickly turn green, yellow and red as the air is accelerated into the low pressure area (remember, low pressure equals high speed, so the reddest area has the highest local flow speed and the lowest pressure). In subsonic flow, the suction peak would be somewhere between 20% and 30% of chord, and the colors would slowly change back to yellow and green if you move further downstream. Now we have local supersonic flow (everything redder than light orange is supersonic here), and instead of slowly decelerating, the air is speeding up to a maximum Mach number of 1.23 at almost 60% of chord length.
This cannot last, and at some point this supersonic pocket collapses. This happens instantly in a shock, and as you know, in a straight shock density increases suddenly and speed decreases such that the Mach number after the shock is the inverse of the Mach number ahead of the shock. In the picture above, boundary layer effects create a lambda shock, which has its name from the Greek letter which looks like the shock pattern here. Aft of the shock, you have subsonic flow again, and a much thicker boundary layer which moves very slowly (blue shade). This is due to the energy conversion through the shock, which converts kinetic energy into heat. But the flow is still attached - even this shock did not cause separation.
If this pressure rise is big enough, the boundary layer will come to an instantaneous standstill, and then the flow separates. This is the shock induced separation you asked about. Unfortunately, the picture above is the best I could find, and I have none with separated flow aft of the shock. But it helps to show that the center of pressure moves aft. This causes a strong nose-down moment. Also, with higher subsonic Mach numbers directional stability decreases. Now even more nasty things can happen: The location of the shock might move forward and back. This changes the size of the supersonic area, causing lift changes. On a horizontal tail, this will also cause pitch changes. If you change elevator position slightly with such a shock on the horizontal tail, the lift change could be severe and in the opposite direction of what you would expect. This causes total loss of control, just when you need the control surfaces to counteract the Mach effects mentioned above. Also, the shock position could oscillate, causing a buzzing sound and, if you are really unlucky, coupling into an elastic eigenfrequency of your structure, resulting in flutter. Not only on the tail surfaces, but also on the wing, affecting the ailerons as well. Now you can start to see what scared the early pioneers about flying near Mach 1 and why they spoke of a "sound barrier".
If you fly fully supersonic, this effect goes away because now the shock moves to the trailing edge and stays there. Now everything will be calm again because the shock location stays fixed. This effect was first experienced and survived on April 9, 1945 by Hans Guido Mutke in a Me-262, which briefly flew fully supersonic in a dive. However, even in fully supersonic flow separation is possible, but then because the air flow will not bend more than what can be caused by total vacuum. In hypersonic flow the density changes become so severe that pockets of "air" are possible which do not contain any air, but a vacuum. But this is more an academic case, except for reentry vehicles with a blunt, rear-facing base.

Ok. I'm a professional software engineer by-trade (and pulling no punches, a damn-fine one), and I have to say, I'm overtly jealous of the vernacular of aeronautical engineers. You guys have the coolest damn terms. I could read your stuff for hours (and, shockingly, understand most of it, but that I attribute to the physics and math training of long-ago). Nice writeup.

@WhozCraig: Thank you for the really nice comment! In order to improve the post, could you please point out what is not easy to understand? I would like that readers understand all of it, and I appreciate your help! Thankfully, Stack Exchange allows to edit posts, so I can make it better.

The fourth paragraph took a few passes before it started to finally gel. The rest was cake to sink in, but that paragraph took some doing. I know its a hard subject to describe, particularly to people that are foreign to most of even the basic concepts, but you did pretty darn well, I have to say. If you changed anything, it would be honing that one paragraph to the less educated masses, but I'm not convinced that would be your target audience to begin with.

I have to admit that I have no easy and intuitive explanation for the differences between subsonic and supersonic flow. It took me a while to explain pressure around an airfoil without vortices and all what comes with the totally unintuitive potential flow theory. I am still working on understanding supersonic flow myself.

Just curious to know which software did you use for the simulation and more importantly which assumptions and computations types have been used (Euler, RANS, turbulence model and so on)? Explanations are really clear to me, nice job (even though I do have some background in that field which might help to grasp the thing)!

Where you describe the issues with supersonic airflow over the elevator, I would only add that, back on the wing, the same issues arise with the ailerons.

... and on the rudder, just when directional stability vanishes. Thanks for the hint.

@Ludivic C.: I took the picture from an academic paper which I cannot locate right now. I would think it is a NS algorithm, but do not know for sure.

@PeterKämpf Look up Awesomesauce: this is it! Very well done.

Why does this happen at Mach Speed?  ...can it happen at lower speeds?

It can also happen at lower speeds, it depends on how the wing is designed and the characteristics of the airfoil (camber thickness profile).
At transonic speeds (0.7 - 1.0 Mach) you can have portions of the airfoils in a supersonic region, meaning that you will have a shock front over (and maybe under) your wing. If the shockwave is strong enough, the flow behind it will be (partially) separated.
When the aircraft reaches Mach 1 the presence of a shock is guaranteed.

Image from wiki
EDIT
In the comments more questions have arisen, I will try to address them.

why does the Shockwave appear?

Short answer: To go back from supersonic to subsonic flow. Supersonic flow is hard to decelerate without a shock, since the air molecules don't "know" what is ahead. The speed of sound is also the speed of small pressure changes, so any signals of what is coming will not reach the air ahead of the shock wave. The air streams along, blissfully unaware of what is coming, until things cannot be maintained and change with a bang.
When the shock wave is so extended to reach the ground, it is called Sonic boom: see the section Sonic boom and sound barrier
See also this NASA article on normal shocks.
Because of friction, the air molecules next to the body have no velocity relative to the bodies surface. The molecules a bit further away will be able to move, but because of friction with the molecules attached to the body, they will be slowed down.  This phenomena is called a boundary layer. In normal conditions this is the way roughly half of the profile drag is created. The other half is pressure drag. In separated flow, friction drag disappears, but since the separated air mass is at a lower pressure than static pressure, and because it sits on the backward-facing part of the airfoil, its pressure drag contribution is massive.

why would the shockwave create turbulence?

Because is an anisotropic event, it is chaotic in nature, it increases the entropy of the air molecules.
Temperature, density, pressure and velocity change by such large amounts at the shock wave (depending on the speed of the supersonic flow, with exact quantities given by solving the Rankine–Hugoniot equations) and in such an infinitesimally small space that the flow downstream becomes extremely chaotic and non-laminar.

Can it happen at lower speeds as well (in straight and level flight, ie, not stalled)?

A shock wave can only happen if you have supersonic flow over the wings. Flow separation happens at all speeds. In the end, all flow will separate at the trailing edge.

It happen on mach speed because that is when the wing will outspeed the pressure wave of it trying to cut through the air resulting in a shockwave when the air slows down again to subsonic speeds this shock is called a recompression shock.

The shockwave is the cause of the separation. And as you see it can happen at lower than 1 Mach. The minimal airspeed at which supersonic flow happen is called the critical mach number. But the speed at which the drag cause by the shock wave become significant is the drag divergence mach number.


text:
Do bigger aircraft make a bigger sonic boom?
I was watching an episode of MythBusters where they were trying to break glass windows and cups using a sonic boom generated by a F/A-18 Hornet, flown by the Blue Angels.  In summary, they were unsuccessful at creating a boom that could shatter the windows, despite a mach speed pass within 200ft of the target building. In the last two passes they even flew straight at the building to focus the boom straight at the house, still no windows were shattered. **
But I've wondered, ever since, if they were unsuccessful because the plane was simply too small.  What if the sonic boom had been generated by a B-1 Lancer or perhaps Concorde, which are several times larger than the F/A-18?  Logically it seems that since you are displacing more air, the energy wave should have more energy.  Then again, the plane isn't going any faster, so perhaps the surface area of the wave would be larger (because of the larger plane) but you wouldn't actually have any more energy at any given point?
Anyone know if a bigger plan than the F/A-18 would have a higher energy sonic boom?  And could that cause glass to break?
Bonus point:
A mathematical formula showing why, I love those things.
For those curious about what a sonic boom is to begin with, here's a wikipedia article.
**Just to clarify, for those who have seen the episode.  A window is broken during the 200ft pass but they say that it only happened because the frame warped, not because the glass itself shattered.  Their reasoning is that only that one window broke (out of several windows and glass cups in the area), and it had a pretty cheap frame on it.

I don't know for sure the relationship, but I found this: The strongest sonic boom ever recorded was 7,000 Pa (144 psf) and it did not cause injury to the researchers who were exposed to it. The boom was produced by an F-4 flying just above the speed of sound at an altitude of 100 feet (30 m)

And here is some really good information

@SSummer - Thanks, it looks like that got spun into a full answer below.  Reading that makes me wonder how much overpressure that F/A-18 was producing at 200ft...  They don't really say in the show.

It would have been hard to use a Concorde for that episode, given that the last ones had been grounded in 2003, six years previously.

The short version is that yes, larger aircraft create bigger sonic booms, but you still have to be flying very low (less than 100 ft.) and really trying hard in order to damage glass.
A sonic boom is so complicated that you will not find a simple formula to determine the strength.  This article describes a modeling tool called PCBoom3 which can be used for sonic boom calculations.

According to several different sites, like this one and this one, the size of the aircraft does affect the intensity:

Factors Affecting Sonic Boom Intensity
The intensity of sonic booms are affected by:

Weight, size and shape of the aircraft.
Altitude
Attitude—orientation of the aircraft’s axes relative to the its direction of motion.
Flight path.
Atmospheric and weather conditions.

As the size and weight of the aircraft increases, the intensity of the
  sonic increases. This is because a larger aircraft displaces more air,
  and a heavier aircraft needs a greater force of lift to sustain
  flight. Thus creating a louder and stronger sonic boom.

and

General Factors Associated With Sonic Booms
There are several factors
  that can influence sonic booms -- weight, size, and shape of the
  aircraft or vehicle, plus its altitude, attitude and flight path, and
  weather or atmospheric conditions.
A larger and heavier aircraft must displace more air and create more
  lift to sustain flight, compared with small, light aircraft.
  Therefore, they will create sonic booms stronger and louder than those
  of smaller, lighter aircraft. The larger and heavier the aircraft, the
  stronger the shock waves will be.

NASA says that a typical sonic boom for the F/A-18 is about 1.4 psf and a different NASA page goes on to say:

Overpressure Sonic booms are measured in pounds per square foot of
  overpressure. This is the amount of the increase over the normal
  atmospheric pressure which surrounds us (2,116 psf/14.7 psi).  At one
  pound overpressure, no damage to structures would be expected.
  Overpressures of 1 to 2 pounds are produced by supersonic aircraft
  flying at normal operating altitudes. Some public reaction could be
  expected between 1.5 and 2 lb.  Rare minor damage may occur with 2 to
  5 lb overpressure.
As overpressure increases, the likelihood of structural damage and
  stronger public reaction also increases. Tests, however, have shown
  that structures in good condition have been undamaged by overpressures
  of up to 11 lb.  Sonic booms produced by aircraft flying supersonic at
  altitudes of less than 100 feet, creating between 20 and 144 lb
  overpressure, have been experienced by humans without injury.
Damage to eardrums can be expected when overpressures reach 720 lb.
  Overpressures of 2160 lb would have to be generated to produce lung
  damage.
Typical overpressure of aircraft types are:

SR-71: 0.9 lb, speed of Mach 3, 80,000 feet
Concorde SST: 1.94 lb, speed of Mach 2, 52,000 feet
F-104: 0.8 lb, speed of Mach 1.93, 48,000 feet
Space Shuttle: 1.25 lb, speed of Mach 1.5, 60,000 feet, landing approach

They also include a graphic that shows the difference in sonic boom between the F-18 and the Concorde, with the Concorde having a much greater sonic boom at the same alittude:

This article specifically covers glass:

Glass. Poorly mounted, undamaged glass in the greenhouse was chipped
  by impact against nail holding points at a sonic boom overpressure of
  12.1 psf. The same type of glass, which was already damaged, was further damaged at a designed overpressure of 7.9 psf. A large
  one-ninth of an inch thick window, intentionally precracked from
  corner to corner, was further damaged by booms of an average 6.5 psf
  overpressure.

So typical sonic booms generated by normal aircraft aren't going to create anywhere close to the 12.1 psf that they say is needed to chip "poorly mounted" glass, but the strongest sonic booms can get well over this.

Sounds to me like they should have tried a pass at about 50ft...  If such a thing were legal.  Though it would be interesting to see if perhaps a B1, at the same speed and altitude, would have created enough over-pressure to break the glasses that were sitting on the table in that episode.

Fantastic, great answer, yet again :).


text:
What legal actions would be taken against a drone pilot if they flew a drone into class B airspace?
Let's say someone has bought one of those quad copter camera UAVs and wasn't completely aware of FAA rules regarding airspace and where they can fly their drone, etc.  Then let's say that they flew it into the airspace of a Class B airport by accident.1  What would happen?
I assume the airport, if they knew about it, would shut down the airspace until the drone had cleared (kind of like that recent fire.)  But after that, what happens?  Does the FAA have a way to figure out who was flying the drone?  And even if they do figure it out, is their an FAA based penalty for flying into controlled airspace with an unauthorized drone?

1 Here's an example of a Class B airport, for anyone who isn't sure what that is: Lambert-St. Louis International.  So, basically, a really big airport.

For those who seem to think this question is a dupe of "Is it legal to fly a Quadrocopter/Drone into an Airport?  What will happen?":  The only fragments that kind of deal with my question are as thus:
From dvnrrs regarding legal action:

Were you to actually fly it over the fence, you could be charged with
  trespassing. If you had any intent to cause harm, other charges might
  apply (up to attempted murder if you were trying to bring down an
  airliner).

From ratchet freak regarding what the airport might do:

When the tower sees unauthorized entry of a drone they will clear the
  airspace and send security to catch the guy controlling it.

Both of these comments lack detail, and that's probably because both of those snippets are from answers that were mostly dealing with the safety aspects of the question rather than with the legal aspects (eg., could a drone actually cause damage).  I am after the legal aspect, and I am looking for more detail than "It's not legal, you may get charged" plus "they'll send security."
CGCampbell did start down that road by listing a statute that contains more detailed rules about what you can do, but it doesn't say what will happen if you don't follow said rules.
I want to know what laws the FAA will use to prosecute, how they will track down the perpertrator and, lastly, what evidence they will need to either fine or charge them with a crime.

The FAA may not have a way but the NSA surely does - everybody who googles this question is a suspect.

@DeerHunter Darn it, you caught us.  This is really just a honey pot for would be accidental law breakers ;).

@Pondlife that question doesn't answer how they would find you, nor does it address what the FAA would do, rather it takes it from the Airport (property owner's) perspective and talks about trespassing.  Not quite what I'm after.

Seriously, did anyone actually read through the question and my reasoning before marking this as duplicate?

@JayCarr I did read the 2nd half of your question but I still thought it was a duplicate.The title of the other question asks 'is it legal'. Though it is true that the body of the question asks more about safety aspects than legal ones, but it does also ask about legality. If the answers don't go into enough detail, that's a different issue. Editing either your question or the other question (to just focus on safety) so they don't overlap would probably help. Just my opinion. The review history also shows people seem divided between Leave Closed and Reopen.

@DannyBeckett Asking for more detail absolutely differentiates a question from another that is asking a broader, less detailed question.  Otherwise this site would only need 1 question: "Can anyone explain airplanes to me?"  All of our other questions are simply deeper version of that very question.  The "dupe" question is not as specific, rather keeping things in broader terms that cover more ground than I do, therefore, they are different.  Not horizontally, but vertically.

I don't know about the US or the FAA but there is always the possibility to resort to old-fashioned police work. There were a string of cases of drones seen flying over nuclear power plants in France last year. The local aviation authority had no easy means to find out who was flying the drones but the police tried to follow some of the drones, lock the area, search cars, etc. They even arrested three people found with a drone in the vicinity of a power plan but I think they did not have enough to link them to previous flights and still haven't figured out exactly what was going on.

Side question -- could a UAS be cleared into a Bravo, provided the operator was in two-way radio communcation with ATC and the UAS was fitted with a working mode C transponder?

@UnrecognizedFallingObject Excellent question, I'd try asking it as a regular question.  I'd be very curious to see the answer.

The answer is somewhat unclear, as there is limited (but emerging) case law and regulation on the subject and the outcome would depend heavily on the details of the airspace violation. However, there are some principles that we can use to shape our understanding:
The FAA has jurisdiction over anything flying, including UAS
This was most recently restated in the NTSB decision in FAA v. Pirker. Combine the definition in FAR 91.1: "Aircraft means a device that is used or intended to be used for flight in the air" with the Applicability section in FAR 91.1(a):

...[T]his part prescribes rules governing the operation of aircraft (other than moored balloons, kites, unmanned rockets, and unmanned free balloons, which are governed by part 101 of this chapter, and ultralight vehicles operated in accordance with part 103 of this chapter) within the United States including the waters within 3 nautical miles of the U.S. coast.

We see then, that anything flying is regulated - either under part 91, 101, or 103. Since UAS are not regulated under part 101 or 103, they fall under Part 91. Finally, the FAA has authority to impose a civil penalty for violations of the FARs under 49 U.S. Code § 46301. Note that an FAA certificate is not necessary for the FAA to assess a civil penalty: in the Pirker case, the FAA assessed its penalty on a Swiss citizen who did not hold a FAA certificate.

Finding the operator of a UAS is difficult
Finding the operator of a UAS is much harder than finding the operator of a piloted aircraft. The FAA was able to find and assess a penalty to Raphael Pirker because he posted a video he made flying the UAS online. In most situations where UAS were where they shouldn't have been, the operators either were never found or turned themselves in:

UAS crash-landed on the White House grounds
UAS interrupts wildfire-fighting activity - a reward was offered for identifying the operator
UAS reported by pilots on approach to KJFK

Basically, if you're flying a UAS far enough away from yourself that a police officer can't see the remote in your hands, you're unlikely to be found if just your UAS is detected. UAS for personal use do not currently require registration or external markings. A recent research project proposes a tracking system - but would require "affixing radio-frequency detection tags to [UAS]."

Actual enforcement would vary
After all of that, what would happen if you did violate controlled or restricted airspace and were caught? This area is pretty fuzzy, and would depend strongly on a number of criteria, including severity and intent. The FAA's toolbox for airspace violations is broad, but is generally tuned toward certificated pilots. A certificated pilot who accidentally entered class B airspace might get anything from a slap on the wrist, to a required training session with an instructor, to a short-term suspension of their certificate, based on the situation: an overwhelmed or confused pilot would get more leeway than one who just wasn't paying attention or was reckless in their navigation.
Suspension and required training are not really in the FAA's toolbox for folks without a certificate, so you likely would expect some sort of monetary fine. The FAA is limited to $10,000 in its fines, so you could not be charged more than that, and could well see something less.

Prevention is the best cure
Some UAS manufacturers are building in safety restictions on their aircraft to prevent them from flying near airports (note that, at least in the linked situation, the restricted areas are less restrictive than B airspace, so would not necessarily keep you out of that airspace.)
The FAA is also working on a mobile app which should easily allow UAS operators to determine areas to avoid. This is relevant to the enforcement situation: if you enter airspace without clearance, but the app said it was OK, that would be a mitigating factor. More to the point, the app would provide information about the status of nearby airspace, allowing you to avoid violation, and the associated penalty, altogether.

If the UAS causes an airport to be closed, even partially and for a short span of time, and it causes expenses (for example an airliner having to keep airborne for several extra minutes), the operator is likely to have to pay for the expenses incurred by the airline, the airport, and so, and that could amount a lot more than the FAA fine. I'm right?


text:
During an engine failure in Diamond DA-40, should the prop lever be at fine pitch or coarse pitch?
Chapter 3 of AFM doesn't mention anything about prop lever setting during engine restarting for both windmilling as well as stationary propeller

If your engine fails in a DA-40, you've lost oil pressure, so there likely isn't enough to push the blade to the coarse position.
A DA-40 doesn't have an accumulator like a Seminole, for example.  So the blade position is already a fine pitch before cranking the engine during start, and the oil pressure allows the blade to be pushed to the coarse position, not the fine position, which is also the reverse of my exemplary Seminole, which needs the oil pressure to maintain a fine pitch.  The Seminole's shutdown-default is coarse for the same reason the DA-40's is fine: a lack of oil pressure.
The likely reason the emergency procedures don't specify, is the system isn't designed to allow you to feather or even coarse the propeller without the oil pressure generated by the engine, not that a DA-40 can feather at all, and you wouldn't have much oil pressure if the engine failed.
At best, the windmilling propeller would turn the oil pump, which would provide some oil pressure, which would in turn coarsen the propeller to some degree, but as soon as the propeller RPM drops further due to the resistance generated by the coarser pitch, so would oil pressure with it, and that would lead to the propeller going toward a more fine pitch again.
To maximize the chance of getting a good restart, allow the propeller to windmill by placing the propeller lever at the fine pitch, high RPM, setting.
Here is a diagram of the system hosted online.  You can see that added oil pressure to the piston causes a more coarse position of the propeller.

Just a sidenote: it is somewhat confusing that DA40 AFM  in-flight engine trouble part does have action item to check the prop lever. What the hell is that all about? To check if it is still there? I mean if it does nothing when engine is out, the why bother checking it

Maybe it is talking about how a low prop RPM setting can increase cylinder head pressures and make the engine run in an unusual manner that can cause odd vibrations in the plane.

That would make sense, common action item is to set prop on high rpm in any kind of predicament. Why the wise men at Diamond decided to tell the stressed pilot in trouble "hey man check out the prop lever" is just beyond stupid if one asks me.

One reason to check the prop lever is to make sure you didn't accidentally pull it back and kill the engine when you meant to be leaning the mixture. These engines are so reliable (properly maintained) that accidentally killing it with the prop lever is a valid possibility (especially if the strap of your passenger's bag accidentally pulled it back- or the mixture).
Checklist=bullet points reminding you to do what you learned in training.
Vy, trim, find a field, head toward it, determine wind direction, plan a downwind, flow check: "fuel, fuel, fuel, spark, air, prop", radio, checklist.

Generally you want full coarse, to feather the prop for minimal drag.  Thinking about it the other way, full fine would be showing the flat of the prop blades into the relative wind, for maximum drag.
EDIT:  I stand corrected.

Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.

On a multiengine airplane you do want to feather the dead engine's prop to reduce drag, but on a single-engine you need the prop to keep windmilling as you troubleshoot and attempt a restart.  This is the reason why, with a loss of engine oil pressure, a multiengine aircraft feathers while a single-engine does the opposite.
After you have completed troubleshooting, tried everything and cannot restart, feathering may be an option if you have enough oil pressure from the windmilling prop/engine to do it.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is required to get back into aviation?
I stopped working as an airframe & powerplant mechanic back in December 2010 to pursue other goals. I still have my A&P but have not been active in aviation since then, I’m considering going back but I have moved twice and have not done anything with my license. What steps are required for me to get back into aviation aside from reporting my new address?

Hi EMore, welcome to ASE. A&P would stand for what?

Airframe and power plant license. It’s required for aircraft mechanics (far part 65)

Last I knew, and I have not worked in the industry for many years, the A&P certificates do not expire, however if you have not worked in field for more than 24 months, you are no longer considered current.
You will need to work under the supervision of another A&P license for 6 months to regain currency.


text:
How does flight control surface authority change with AOA?
I am working on semi-realistic (let's just call it "believable") flight simulation component set for a game engine. One of the topics that I tried to research but could not find reliable information on is the authority or effectiveness of flight control surfaces over AOA. I would prefer to see some graphs or some approximate math expression. I've found more or less enough information on other topics (e.g. lift coefficient over AOA) but I cannot really come up with reliable information (even just theory) regarding flight control surfaces. Gut feeling tells me that they would probably behave similar to wing, but on the other hand elevators should really not lose much authority even past stall AOA.

I think you may be misunderstanding the dynamics, the effectiveness of flight controls varies with airspeed. Maybe what you are asking is how much force is generated by flight surfaces at different angles?

Control authority is a function of the local dynamic pressure (the product of air density and speed squared), geometry (the product of control surface area and lever arm), local angle of attack $\alpha$, the angle of deflection $\eta$ and the relative flap chord. Generally, the control surface works fairly linearly in a range between $-15° < \eta < 15°$ for a relative flap chord of 25% rsp. $-25° < \eta < 25°$ for a relative flap chord of 15%. Interpolate for values in between. Note that flap effectiveness is proportional to the square root of the relative flap chord, such that a flap of 15% needs twice the deflection angle to effect the same control authority than a flap of 60% relative chord.
Bigger deflection angles reduce flap authority, and here it becomes complicated, because local flow phenomena can make a big difference, up to control reversal. For a simple model you may just assume that control authority is cut in half for all deflections exceeding the limits mentioned above.
This all is only valid for moderate angles of attack, when forces change linearly with angle of attack changes. Once you approach the lift force limits of the particular control surface, authority suffers. Now it is important to know the local flow field: Elevators fly in the wake of the wing, which in effect reduces the angle of attack changes at the elevator compared to those at the wing. This means that most elevators still have a healthy lift margin when the wing starts to stall. But stall they will, eventually. To give you an idea how a wing performs over the first 180° of angle of attack, see the plot below. A plot for a control surface should look similarly. It is taken from a book which should exhaustively answer all your questions about control surface authority, be it subsonic or supersonic, for all conceivable configurations. It has been written by Sighard Hörner half a century ago and is still the best you will be able to find on this topic.

For an example of an airfoil's flap effectiveness over AoA, see the plot below of the XFOIL calculation for a Wortmann FX71-L-120 symmetric airfoil with a 25% flap at a Reynolds number of 1 Million and with a fixed boundary layer transition at 60% of chord. Note especially the hysteresis close to the maximum and minimum lift angles of attack at higher deflection angles (own work; c$_a$ is the lift coefficient).

ALL flight control surfaces lose authority at lower speeds and/or in less dense air (heat/altitude). In most cases, this can be compensated somewhat by increasing the deflection until the max is reached.
Some aircraft with fly-by-wire can compensate for the 'sloppy' controls somewhat by doing the increase in deflection automatically.
Although I can't help you with formulas or graphs, I can tell you that the reduction in effectiveness can be very significant.
However, by design, the elevator stalls much later than the main lifting surfaces for stability reasons and to have the ability to get out of a stall (naturally or by pilot input).


text:
How does email communication with a private pilot affect FAA rules on compensation?
If a person exchanges email with a private pilot (who normally operates under FAR 91) with a view towards hitching a ride or taking an empty leg flight, does this act mean that the pilot must then operate under FAR 135?
Or alternatively, if there is no discussion of payment and/or barter, would the ride still take place under FAR 91?  I have watched the demise of AirPooler, but this was a much more structured arrangement which blatantly  invited scrutiny from the regulator.  This question assumes there is no intermediary like AirPooler or BlackJet.
I am trying to find out what actually triggers an escalation in the rules.  FAA would be fine, but anything on European regulators would be massively welcomed.  I am UK based, not a pilot, and use empty leg on occasion, so I might be affected if the European regulators follow the FAA's lead.  Knowing the trigger point is key.

Is your question assuming that no form of payment or compensation is taking place?

In most cases, assume no payment or compensation. Other times are bartered for services.

That's two completely difference scenarios though, which is why I ask.....

If the trigger point differs according to scenario, we can scope this one to no payment and I can ask another question if needed.

The system that we use here expects questions that can have one correct answer, while you are asking for two different answers (FAA and EASA).  Since you already have the FAA answer, I would suggest asking a different question about the EASA rules.  For more information, see [ask] and FAA -vs- EASA meta discussion.

@Lnafziger, thanks for the tip and the META link. I will check it out.  I am not sure another question is really necessary at this point. At the moment it has had 140+ views which may be sufficient exposure. If it runs for a full 24 hours, then I'll certainly edit the question to remove EASA and accept the answer below.

Well, if you are still interested in the EASA answer, that would be the way to go.  Then, you could accept an answer on both questions, rewarding the person who put effort in to the answer that helped you in both areas.  There are some other discussions on meta as well about different jurisdictions and how to handle them, and the consensus was that we should have one question per governing body so that we don't end up with many different answers to the same question and no way to mark them all as correct.  Think if people want to know next about Russia, China, Australia, etc, etc...

I'll answer it from the FAA since that's what I know.
The FAR is pretty clear on the matter here

(a) Except as provided in paragraphs (b) through (h) of this section, no person who holds a private pilot certificate may act as pilot in command of an aircraft that is carrying passengers or property for compensation or hire; nor may that person, for compensation or hire, act as pilot in command of an aircraft.

However if you want to split the cost evenly, you can:

(c) A private pilot may not pay less than the pro rata share of the operating expenses of a flight with passengers, provided the expenses involve only fuel, oil, airport expenditures, or rental fees.

For what it's worth I would say you can still operate under Part 91 in the case you have outlined.
Basically the FAA prevents a PPL holder from receiving any kind of compensation for a flight. This Article talks about some of the points that the FAA discussed when shutting down these services. The issue with some of these sharing services is that the middle man (AirPooler) and the like take a fee, this seems like the aspect that the FAA had an issue with since although the pilot was not profiting per se, someone was, this seems to be the FAA trigger. I don't know much about the deep aspects of the law but I'm sure there are things that can also arise with being a middleman/deal broker of these kinds of things.

+1, would be grateful for a view, supported or otherwise, on this side of the pond also.

As for the other side of the pond, I don't know the regulations (I'm a good old yankee) so I will refrain from commenting.


text:
What is considered "high altitude"?
The AFM for DA40 states in 4A.3.9 CRUISE checklist:

Electrical fuel pump . . . . . . . . . . . . . . . . . . . ON at high
  altitudes
CAUTION Operation at high altitudes with the electrical fuel
  pump OFF may cause vapor bubbles, resulting in intermittent low fuel
  pressure indications, sometimes followed by high fuel flow
  indications.

I (sort of) understand the reason for needing to compensate for lower ambient pressure with a mechanical fuel pump, but the problem is I can't find a clear reference to what is considered "high altitudes" for this purpose? It has a demonstrated operating limit of FL164, so we're obviously not talking crazy high.
Is it more of a "if you experience these symptoms, try turning on the fuel pump" type of caution? Or is there some specific altitude above which the pump should always be on?

Personal opinion would be that about 10000' ASL would be about "High Altitude" because you definitely need a pressurised cabin above that.

Pressurization is not a requirement for any aircraft, only for certain operations.

I considered whether oxygen requirements might be a determining factor for "high altitude", but oxygen requirements are based on the effects of low air density on human breathing (ability for oxygen to reach our tissues) which does not necessarily have any bearing on how well the fuel pump works and the likelihood of fuel vapor forming. i.e. That I require supplemental oxygen at FL125 doesn't mean anything to fuel pump.

After further analysis, it looks like boiling point (temperature at which the vapor pressure equals the ambient atmospheric pressure) is a key issue related to altitude. Avgas is formulated with a low (6.5 psi) vapor pressure to reduce risk, but above a given altitude vapor bubbles can begin to form. An additional concern is the solubility of gasses in fuel which increases inversely with pressure and can lead to fuel pump cavitation.
Question I still haven't been able to answer is - what is the critical altitude, and is it density altitude or pressure altitude to be concerned with?

@SMS von der Tann: Why do you think you need a pressurized cabin above 10K ft?  I routinely cruise at 11K/12K in my Cherokee, since there are mountains hereabouts that tall.  (Of course I routinely hike & ski in those mountains, so I'm fairly well adapted to the altitude.)

Pressurization and supplemental oxygen requirements are often confused. Above 10,000 MSL it is required for Part 135 operations to utilize oxygen, whereas for Part 91 the requirements begin at 12,500 MSL. For these, pressurization eliminates the oxygen requirements below certain altitudes. Transport category planes do have their own pressurization requirements, which I think require cabin pressures below 8,000ft at all times.
In any case - none of that is particularly relevant to fuel vapor formation and the use of electric fuel pumps.

It seems I've asked a question that has no answer. From the explanation below provided by a Diamond Aircraft representative, it is clarified that the intent of the AFM's caution is not to always run the electric fuel pump at high altitude, but to use it as needed to clear the symptoms - which are more likely to occur as altitude increases.

The IO 360 with the Precision Fuel Injection System does not have a
  vapor separation and return line to the tank(s). Vapor in the fuel
  system will travel through the fuel system including flow and pressure
  transducers, into the injection system. Vapor can form under higher
  ambient temperatures and low ambient pressures. Fuel gravity fed
  through the system (electric pump off) from the tanks is “lifted” by
  the mechanical pump. The suction of the mechanical pump can cause some
  cavitations (vapor formation), especially as the fuel flows through
  resistances such as elbow fittings. Vapor bubbles will cause momentary
  low pressure indications and may also cause momentary erratic or high
  flow readings as the flow transducer turbine spools up with the loss
  of fluid resistance. These momentary occurrences can be suppressed
  with the electric pump. The electric pump activation increases system
  pressure to compress or return to liquid, the formed fuel vapor
  bubbles.
[...]
The intention of the steps for electrical pump selection, above each
  of the Cautions in the AFM is not for continuous operation of the
  pump, as this could potentially mask a defective mechanical pump and
  eliminate the redundancy of the fuel delivery system. If the abnormal
  fuel pressure or flow indications do not stabilize with the electric
  pump on, emergency procedures and maintenance action may be required.

I remain curious about the relationship between Avgas vaporization and altitude. Does bubble formation increase linearly with altitude? Alas I'm not sure I'll ever have an answer to that question, and it does not appear that an answer would change my operating practices regardless.

Good question and good on you for contacting DA to get an answer! Thanks for coming back to share it with us.


text:
What is an afterburner and how long can a jet fly on afterburner?
I heard this word so many time in military documentary about jets, especially fighter jets, but I don't fully understand what it is or how it works. In a documentary from the Discovery Channel, all but one fighter needed to engage afterburner to reach supersonic speed. Could you tell me why is that?
The documentary also states that a fighter jet rarely engages this system. If the afterburner system can make a jet fly faster, why don't they use it all the time? Is that because of the airframe cannot handle supersonic airflow for a long period of time?

This Wikipeadia article tells you everything you need to know

There are a lot of separate questions here, and I think this should be broken up into more than one question. Also, I'm surprised it's not a duplicate but I can't find anything.

Because of the extreme fuel consumption, afterburners are typically used for rapid acceleration, or air-to-air engagements, but not sustained flight.

*How long can a jet fly on afterburner?" Until the fuel runs out

An afterburner is a secondary combustion system which burns additional fuel downstream of the combustion chamber, to further increase thrust at the expense of much higher fuel consumption.
This is the Pratt & Whitney F100 afterburning turbofan, variants of which power the USAF's 4th-generation fleet of F-15s and F-16s:

The final spoke-looking thing just beyond the turbine fins, plus all the space between the turbine core and the exhaust nozzle, is the afterburner. In this area, fuel is sprayed directly into the exhaust stream from the turbine core, where the heat from the air leaving the core is enough to ignite it. This additional pressure adds to the thrust produced by the turbine.
As I said, though, the tradeoff is increased fuel consumption, sometimes usually dramatically so. The F-16 at full military power and low altitudes burns about 8000 pounds of fuel an hour, which with a full droptank configuration gives it about 2 hours' flight time. Cruising at higher altitudes, that flight time can be further extended as both the higher altitude and the lower throttle setting (about 80%) reduce fuel flow rate by up to 40% versus low-altitude flight.
In full afterburner at low altitudes, the F-16 can burn in excess of 64,000 pounds an hour. At full throttle, a U.S.-variant F-16 with maximum external fuel stores has about 20 minutes until it's on emergency reserves (which would only last an extra minute or so at full afterburner). The speed gain is minimal; the F-16 cruises at between 450-550 knots, while full afterburner only increases that to about 700-800 knots with a typical underwing loadout. So, burning 8 times the fuel, you get around a 50% speed boost.

Thank you for a detailed explanation. I know this is kind of out of the question, but how does the F-22 can go up to 1.5 times the speed of sound without afterburner?

The F-22 can supercruise due to a number of design factors. The two big ones are the airframe's internal weapons bays which allow the aircraft to be mission-ready with no drag-producing external munitions hardpoints, and increases in engine performance envelope through use of variable bypass (the engine can switch from a low-bypass turbofan to a pure turbojet at higher altitudes and airspeeds where the pure jet is more efficient). The F-15 and F-16 can supercruise - barely - in a clean configuration, but that would be of little use in combat as the only internal weapon is the Vulcan 20mm.

Oh! I though the F-22 can do such a thing mainly because of powerful engines and nothing more. So, by making a stealth aircraft, we can get a nice benefit of less draft which leads to less fuel consumption and higher speed. Nice to know.

Also using the afterburner often results in the engine needing to be trips out and rebuilt!  Still better then getting shot down!

"The speed gain is minimal" <= Unless you're trying to outrun a missile.

@Mark - Well. trying to outrun one is a fool's errand; the AMRAAM flies at Mach 4.5 and even short-range IR missiles easily exceed Mach 3. Where afterburners help is in giving the pilot enough energy for a max-G turn at the critical moment to "out-turn" the missile. Even then you won't want the speed per se (the F-16's best turn rate is around 320 knots and its minimum turning radius is at even lower speed), but the thrust to maintain your energy through a corner airspeed turn.

@IanRingrose - Are you sure? F-15Es and F-16s with ground-attack loadouts pretty much have to use full afterburner to get in the air. If the engine had to be stripped down after every sortie that a max afterburner takeoff was used, combat readiness numbers would be in the toilet. I could understand the engine needing an overhaul after extended afterburner use such as in a dogfight, but if the airframe's been subjected to max-G turns in a furball, there is a lot more on the plane that has to be stripped down.

@KeithS You are only thinking in terms of A-A missiles, but even there timing can still be important. If the missile is actively guided, and you have a counter shot setup (or in the air) then extra speed might be enough for your missile to catch and hit your target. Additionally, there are a number of SAM Batteries (some still in use by some long-serving dictator-states) that can be outrun, or at least out-climbed. But in a loaded jet (like an F-16CJ) that extra AB speed is going to buy you a few extra seconds of life that just might be enough in a number of situations.

@KeithS, I don't know about the F15/15 but it used to be the case on some aircraft that even using the jet at full power resulted in lots of maintenance being needed.

@IanRingrose Well military aircraft generally need a lot of maintenance anyway. An F-16 costs an average of 22 grand in combined fuel and maintenance every hour it's off the ground. The F-15 costs 45 grand, and the F-22 a whopping $70,000 per flight hour. In man-hours per flight hour, the F-22 is apparently requiring 30 hours ground maintenance for each hour airborne.

Dead thread and a small point of pedantry, but exhaust gas temps are actually not high enough typically to light an afterburner so a spark igniter is used - except on the Blackbird where even that couldn't relied upon with the low-volatility fuel so TEB was used,

Missed the edit timeout, but source is AgentJayZ on YouTube, he mainly works on older turbojets but I presume it's still the case.

Wait, what? 50% is a small increase? Is it because I'm a physicist, not a pilot, that I find that really large? (Of course, for a pilot there is a significant non-linearity of utility in the range close to the maximal speed of SAM and air to air missiles, to say it in physics terms) That said, if the increase is roughly to the power of 5, that is quite a lot.

@VolkerSiegel - The "small increase" is relative to the fuel cost. You're increasing velocity by 50% by burning 800% more fuel. And I'm a mathematician more than a physicist, and to me "increase roughly to the power of 5" is an x^5 increase, which we are absolutely not talking about here.

By using the afterburner, fuel is injected into the downstream of the turbine. Exit velocity gets higher -> More thrust.
Comparison of the generated thrust in an F/A-18C Hornet:

Maximum thrust without afterburner   10,440 daN (each 5'220 daN)
Maximum thrust with afterburner  15,660 daN (each 7'830 daN)

(The F/A-18C Hornet uses 2 General Electric F404-GE-402 turbofans)
Some airplanes need the afterburner to reach supersonic speed because the "normal" use of the jet turbine doesn't generate enough thrust. Using the turbine in "normal" mode (without afterburner) is also called "military power" or "dry". Using the turbine with afterburner is also called "full power" or "wet".
From this wikipedia article:

Due to their high fuel consumption, afterburners are usually used as little as possible; a notable exception is the Pratt & Whitney J58 engine used in the SR-71 Blackbird. Afterburners are generally used only when it is important to have as much thrust as possible. This includes during takeoffs from short runways, assisting catapult launches from aircraft carriers and during air combat situations.

It's true that a fighter jet rarely engage the afterburner because it uses extreme amounts of fuel. Sometimes up to the factor 10 to the normal fuel useage. Thats also why they don't use it all the time: The operating-range of the fighter-jet is drastically reduced by using the afterburner.
The pilot can use the afterburner in different stages to find the perfect ratio of fuel usage / speed / range..
Source (In english): http://www.lw.admin.ch/internet/luftwaffe/en/home/dokumentation/assets/aircraft/fa18.html

I had to look up what a daN is.  For anyone else who is confused, "da" is the abbreviation for the metric prefix "deca" (also "deka"), which means a factor of 10.  (Thanks Wikipedia!)  So 1 daN is 10 N.  1 N (N is the abbreviation for Newton) is the metric unit of force which will accelerate a 1 kg mass at 1 m/s^2, of course.

And as 1 g is pretty close to 10 m/s^2, that means 10 N = 1 daN ~ 1 kgf (kilograms force). That means you can keep using your old numbers and just replace the units, while now "properly using metric". You can also read the numbers 10,xxx daN and 15,yyy daN as "10 tons of thrust" and "15 tons of thrust" instead.

It is possible to design an aircraft that can cruise at supersonic speeds without using afterburners (for example Concorde, the British TSR-2 strike/reconnaissance aircraft, and the Tu-144). The aerodynamic drag force is higher at transonic speeds than when supersonic, and using afterburners to accelerate through the transonic speed range qucker may actually reduce the total fuel burn. That was definitely the case for Concorde. The afterburners were also used to shorten the takeoff roll on Concorde.
Most jet fighters are not designed for "efficient supersonic cruising in a straight line at constant speed," so supersonic flight without afterburners is not the main design consideration.

There is also the famous SR-71 Blackbird which cruises at Mach 3 and faster... The afterburner from the 2 Pratt & Whitney J58 turbines is used very often and long. But this aircraft is designed to operate at high altitudes and at high speeds (up to Mach 3.36)

"Most jet fighters are not designed for efficient supersonic cruising in a straight line at constant speed, so supersonic flight without afterburners is not the main design consideration." That was true until somewhere between generation 4.5 and 5 of fighter design. Supercruise is a design requirement of most of the cutting-edge fighters of the last ten years or so, including the Raptor, Eurofighter, Rafale, PAK FA and Chengdu J-20, even when radar stealth is not a primary requirement.

I flew B-1B's for 7 years.  I've also had flights in F-15s and F-16s.  The B-1 has 4 afterburners, but a lot more gas than the fighters, so I rarely had to stay out of burner due to fuel.  There's lots of reasons to minimize burner usage, though:

Operationally, AB makes you highly visible to all.  At night, you put a spotlight on yourself.  Daytime, everyone on the ground can hear you.  IR sensors will find you quickly and easily, and even lower tech-IR missiles will prefer your burner to flares.

That extra 50% beyond mil power is actually an awful lot. When you do use burner, you don't need it for long.  The B-1 could accelerate in full AB from .8 to .95 mach in just a few seconds.  Operationally, you just don't need AB that much or often.  If you're trying to defeat a missile, you're going to use excess airspeed first to slow to cornering velocity.  The B-1 can maintain cornering velocity without burner since it's at relatively low g.  A fighter at 7+ g will need some burner to maintain energy especially at cornering velocity, but as it can turn 90+ degrees in just a few seconds, it doesn't need much or any burner.  Regardless, in a turn to defeat a radar missile, since IR missiles detect 'passively' meaning there is little or no warning, a pilot will frequently assume there's a heat seeker in the air when turning to defeat a radar missile and will avoid burner anyway.

Close-in air to air dogfighting is one of the few times a combat aircraft needs extended burner.  In fighter combat, energy management is very important.  No one wants to be on the losing end.  Get too low on airspeed, and your jet turns too slow and you lose, so fighter pilots will use whatever burner they need to keep the threat off their tail and win the fight.  In the B-1 as well, in fighter intercept exercises, that was when we tended to use more burner.  We tended to use it to accelerate quickly to complicate the fighter's intercept, and in some cases to bug out with a fighter on our tail.

The other regime where burner use is frequent is takeoff.  This is statistically one of the most dangerous phases of flight, and reaching flying airspeed quickly minimizes the danger.  When I was flying, the B-1 always took off in burner – not sure now.  Fighters can under certain conditions take-off in mil power, but I've rarely seen it.

Burner use in American jets ABSOLUTELY DOES NOT ADD SIGNIFICANTLY TO REQUIRED MAINTENANCE AND DOES NOT HARM THE ENGINES.  The poster who mentioned that might have seen something on the MIG-25, which will destroy its engines in high speed flight.  Presumably, other Soviet fighters have some maintenance issues with burner use, but American combat aircraft are built to employ burner whenever needed without damaging engines.

Altitude is a very important point, as burner fuel flow will decrease with altitude.  In thin air, there is less oxygen available for combustion, so the fuel controls have to adjust accordingly.  As the previous poster wrote, thinner air creates less drag making it easier to go fast.  But... as a commercial pilot today, I've flown with many former fighter pilots, and whenever we get to talking about it, few of us have spent time above 40,000 feet.  The higher service ceiling is a nice stat for the contractor sales teams, but there's rarely an operational reason, and a lot of bad things can happen (like engine stall and physiological emergencies) up in the 40's.

https://www.nasa.gov/centers/dryden/pdf/88117main_H-1449.pdf
Scroll down there's some useful graphs that can give you some idea. AB increases exhaust temperature and thus allows for an increase in exhaust speed. By actuator disk theory this means the in flight thrust at MAX will be closer to the static number than in flight thrust at MIL for any given speed. That's why an F-15 at 40K ft can only fly at M0.95 at MIL but can do M2.5 at MAX with only a 63% increase to static thrust.

The answer is it depends on your altitude. A lot.
For example, I'll take an F-16 since I asked this to somebody that identified himself as a former F-16 crew chief online:  An F-16 flying at full military thrust at sea level consumes about as much fuel as full afterburner at FL400 (40000ft).  At the F-16 service ceiling of FL500, full afterburner will be using quite a bit less fuel than military thrust at sea level.
So way up high, full afterburner might be usable for even 30 minutes if the climb is done efficiently and the big centerline drop tank was used.  That's how the F-16 can actually reach Mach 2. It will take a while on afterburner to accelerate that much.
This also means that full afterburner won't be producing that much extra thrust, but since the air is so thin, it will have a pretty substantial effect in real airspeed obtained.


text:
Why isn't the F-15E called F/A-15?
In my understanding, "F" stands for "fighter" (air-to-air) and "A" for "attack" (air-to-ground). E.g.,

the F/A-18 Hornet can attack air and ground targets, which is why it's called F/A-18.
the A-10 Warthog is used as an attack aircraft, which is why it's called A-10. However, it's also capable of attacking air targets using IR missiles.

The F-15E Strike Eagle is also capable of attacking air and ground targets. So why is the Strike Eagle still called F-15E and not F/A-15?

Interesting fact an F-15 has never lost a dogfight.

@Ethan Not True. What happens when an F-15 fights an F-15 at red flag? What you mean to say is than the F-15 has never been destroyed by an enemy aircraft.

@HCBPshenanigans I mean fighting another type of aircraft.

F-15 regularly loses simulated dogfights at red flag against other types.

@HCBPshenanigans Simulated dogfight $\neq$ dogfight.

They US Navy didn't re-designate the F-14 either after it was modified to carry and use air to ground munitions.

"Not a pound for air-to-ground"

The aircraft you are mentioning are completely different from each other and some of them are not even named according to the US Tri service Naming System.
In general, the aircraft retain their original designation given even though the later versions have little in common with their predecessors.
The F 15 was a direct result of the studies that followed the Vietnam war and the a response to new generation Soviet aircraft like the Mig-25.

"McDonnell Douglas F-15A USAF". Licensed under Public Domain via Wikimedia Commons.
The F-15 was designed purely as a air superiority fighter, with full emphasis on maneuverability and agility with "Not a pound for air-to ground!". The first to variants of the F-15 were called F-15 (single seat) and TF-15 (two seat), which were retrospectively named F-15A nd F-15B after the introduction of F-15C. The F-15E was a strike variant; however, it could defend itself against enemy aircraft and thus retained the 'F' title.

"F-15E Strike Eagle" by Gerry Metzler - IMG_214. Licensed under CC BY-SA 2.0 via Commons.
The case of F/A-18 is different. The initial plan was to purchase three different version of the aircraft, F-18A (Fighter), A-18A (Attack) and TF-18A (Trainer). However developments in avionics and some design changes led to the
first two versions joined together as F/A-18A and the trainer version being named F/A-18B.

"US Navy 050803-N-0295M-148 An F-A-18B Hornet, assigned to the U.S. Naval Test Pilot School, folds-up its landing gear after taking off for a training flight from Naval Air Station Patuxent River, Md" by U.S. Navy photo by Photographer’s Mate 2nd Class Daniel J. McLain
Most of the aircraft in service today are multirole and their names mostly reflect what their intended purpose is rather than their actual purpose. With modern avionics and high precision weapon systems, almost any aircraft could be made a strike aircraft
It is to be noted that the naming convention is followed only loosely in some cases. A good example is the F-117 Nighthawk, which is not a fighter aircraft in any sense of the word.

Good point about the Nighthawk. I've wondered about that before.

It's important to note that not all two-seaters are trainers. These jets were developed at a time where, as you state, avionics were just improving to the point where one pilot could handle the necessary workload. The "B" (and "D") variants of both planes are fully combat-capable, and could have been the primary service variant if the USAF or USN had considered it necessary to have two people flying the plane (like the F-4 and F-14). That's still true, though less common, in newer designs; the Super Hornet comes in one- ("E") and two-seat ("F") variants, and both are in combat service.

First off, "F/A" is a non-standard designation under the tri-service designation system. "FA" or "AF" would be standard; the second letter would, according to the system, be the "primary mission" of the aircraft, whichever of those the top brass felt the design was better at. The Hornet would probably have been designated "FA" as its primary goal was to replace the aging A-4 Skyraider as well as to supplement the F-14.
The F/A-18 has that designation because the Hornet was originally supposed to be two planes, the F-18 and the A-18, which would differ only in the required avionics and in minor differences to the weapons hardpoints. "F/A-18" became the internal shorthand for the project as a whole. When it was demonstrated that avionics improvements allowed a single airframe to satisfactorily perform both mission profiles, the "F/A" designator was applied to the production design.
The F-15E should, most accurately, be designated AF-15E, adding a "modified mission" designation for ground attack (similar to other modified mission designations such as the RF-4 reconnaissance variant of the Phantom, or the current EA-18G "Growler" variant of the Super Hornet). Its parent design was a pure air superiority fighter which was modified to produce the Strike Eagle, mainly by strengthening the wings, improving engine output, adding CFTs for better range without droptanks, and replacing practically everything under the skin forward of the intakes (avionics, radar, etc). The Strike Eagle retains much of the air superiority prowess of its lineage, and has been tasked with "Combat Air Patrol" missions often in addition to a primary strike mission (along the lines of "go here, drop your bombs, refuel at this tanker and then stay on station for an hour before heading back"), but its primary mission is ground attack (interdiction, deep strikes, SEAD, etc).
The main reason why the mission designation of the Strike Eagle was not changed is that the USAF has rarely made the distinction between an air-to-air fighter and a light air-to-ground strike aircraft. Pre-1962, any one- or two-man aircraft developed for the Air Force was a "fighter" (evolved from the older "Pursuit" designation in use through WWII) regardless of its specific mission profile. After 1962 with the tri-service system in place, if the aircraft is designed with air-to-air engagements in mind, it is a "fighter" in the USAF's eyes no matter what else it does. The A-10 is the sole production aircraft designed for USAF use to be given that designation (which is fitting as it's designed from the ground up to kill armor, and typically carries only a pair of Sidewinder-Ms for basic self-defense); all other "A"-designated designs in the Air Force arsenal since 1962 have been Navy planes first, with the Air Force having to adopt the pre-existing designation under the tri-service rules. As the Navy's never really been interested in any variant of the F-15 since fairly early in the VFX program, it's only to be expected that the Air Force top brass would ignore the attack designation here.

The F-15A and F-15B came along first, and they were pure air-to-air.  (The B model is two-seat so an instructor or observer could ride along.)  Then the F-15C and F-15D were built, also as pure air-to-air platforms.  (Again, the D model has the back seat while the C is single-seat.)
Then they got the idea for the "E" model, which has the second seat for the WSO and all the various equipment for ground attack.  But it's still a model of the F-15.  This far into production, it wasn't thought to be necessary to change the designation from F-15 to F/A-15.  Maybe if the "E" had been first and the air-to-air variants had come along later, they might have called it "F/A-15," but not necessarily.  The F-117 was pure ground attack, and it's an "F" not an "A" like the A-10.  The F-16 has lots of ground attack capability, but it's an "F" not an "F/A" designation.
The honest answer is, the Air Force places a low priority on the "purity" of only using the "F" designation on air-to-air platforms and nothing else.

The pilots call the F-15E the Mud Hen (at least some of them do).